This application claims the priority of Korean Patent Application No. 2003-95957, filed on Dec. 24, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a nitride light emitting device (LED) and a manufacturing method thereof, and more particularly, to a nitride LED having an electrode structure of improved emission efficiency and enhanced driving capability and a manufacturing method thereof.
2. Description of the Related Art
Currently, transparent conductive films are used in various applications, including photoelectronic devices, displays, or energy industry.
In the field of LEDs, there has been developed a transparent conductive film electrode for smooth hole injection and high-efficiency light emission.
Currently, transparent conducting oxide (TCO), transparent conducting nitride (TCN), and the like, are most actively researched as transparent conductive film materials.
Examples of the transparent conductive oxide include indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), indium tin oxide (ITO), and the like, and examples of the transparent conductive nitride include titanium nitride (TiN), and the like.
These materials, however, have limitations to be employed alone as a p-type transparent electrode of a top-emitting gallium nitride based LED because they have a relatively large sheet resistance value, high light reflectivity, and a relatively small work function value.
That is to say, the above-noted transparent conductive films, which have a relatively large sheet resistance, that is, substantially 100 Ω/squ may make it difficult for a LED to perform current spreading in a horizontal direction with respect to the LED, that is, in a direction parallel to an interface surface of the LED, during film formation using PVD such as sputtering, e-beam evaporation, or a heat evaporation. In addition, the transparent conductive film makes it difficult to achieve vertical injection of holes smoothly in a vertical direction. Thus, such transparent conductive films are limited in achieving large-area, large-capacity, high brightness LEDs.
Further, the above-mentioned transparent conductive films deteriorate light emitting efficiency because they have high reflectivity with respect to light emitted from the gallium nitride based LEDs.
Next, since transparent conductive films, including indium tin oxide (ITO), titanium nitride (TiN), and the like, have a relatively small work function value, it is quite difficult to form an ohmic contact through a direct contact with p-type gallium nitride.
Finally, when transparent conductive oxide (TCO) is employed as an electrode such that it is brought into a direct ohmic contact with a gallium nitride based compound semiconductor, oxidation of gallium is likely to occur on a gallium nitride surface during formation of a film, resulting in generation of gallium oxide (Ga2O3), which is an insulating material, making it difficult to form a good ohmic-contact electrode.
Meanwhile, in achieving a LED prepared from a gallium nitride based compound semiconductor or a laser diode (LD), the structure of an electrode for forming an ohmic contact between the semiconductor and the electrode becomes quite an important factor.
Such gallium nitride based LEDs are classified as top-emitting light emitting diodes (TLEDS) and flip-chip light emitting diodes (FCLEDS).
In the TLEDS, which are currently widely used, light is emitted through an ohmic contact layer being in contact with a p-type cladding layer. In order to achieve high-brightness TLEDS, one requirement is to form a current spreading film, that is, a current spreading layer, as a good ohmic contact layer for compensating for a high level of sheet resistance of the p-type cladding layer having a low hole concentration. Therefore, it is necessary to provide for smooth hole injection, current spreading, and high light emission performance by forming a current spreading film layer having a low sheet resistance value and a high degree of light transmittance.
The known TLEDS have an electrode structure in which a nickel (Ni) layer and a gold (Au) layer are sequentially laminated on a p-type cladding layer.
The nickel/gold layer structure has good specific contact resistance in a range of about 10−3 to about 10−4 Ω° C., and is used as a semi-transparent ohmic contact layer.
When such a nickel/gold layer structure is annealed at a temperature of about 500° C. to about 600° C. under an oxygen atmosphere, nickel oxide (NiO), i.e., p-type semiconductor oxide, is formed at an interface between a gallium nitride based p-type cladding layer and a nickel layer as an ohmic contact in an island shape, thereby reducing a Schottky barrier height (SBH) and easily supplying holes as multiple carriers to a region around the p-type cladding layer.
In addition, annealing of the nickel/gold layer, followed by forming the p-type cladding layer, removes a Mg—H intermetallic compound for reactivation of increasing a magnesium dopant concentration on a gallium nitride surface, thereby increasing the concentration of effective carriers on the surface of the p-type cladding layer to 1018 or greater. The increased concentration of effective carriers is believed to cause tunneling conductance between the p-type cladding layer and the ohmic contact layer containing nickel oxide, exhibiting an ohmic conducting characteristic with low specific contact resistance.
However, the TLEDS using a semi-transparent nickel/gold film electrode is poor in light emitting efficiency due to presence of a light transmittance suppressing component, that is gold (Au), which is a limitation in achieving next-generation large capacity, high-brightness LEDs.
To increase emission of heat generated during operation of a LED and light emission efficiency, a reflective layer has been employed to radiate light through a transparent substrate made of sapphire in the FCLEDS structure, which, however, has several limitations, including high resistance due to oxidation and poor adhesion of the reflective layer.
To overcome such problems with the TLEDS or FCLEDS structure, transparent conductive oxide having better transmittance than a semi-transparent nickel/gold layer structure used for the existing p-type ohmic contact layer, e.g., ITO, has been proposed as a material for a p-type ohmic contact layer. However, although the ITO ohmic contact layer increases the output power of the LED, it requires a relatively high operation voltage, which is still a limitation as a candidate material for use in large-area, high-capacity, high-brightness LEDs.
As described above, it is quite difficult to develop a transparent electrode for establishing an ohmic contact for several reasons that follow.
First, low hole concentration and high sheet resistance of not less than 104  Ω/squ of p-type gallium nitride make it difficult to form a transparent electrode.
Second, since there is no transparent electrode material having a relatively high work function value than that of p-type gallium nitride, a high schottky barrier is formed at an interface between p-type gallium nitride and the electrode, thereby making smooth hole injection difficult.
Third, most of transparent electrode materials exhibit contradictory electrical and optical characteristics and have a high degree of light transmittance, so that they typically have large sheet resistance, thereby sharply reducing a tendency of horizontal current spreading. Accordingly, a large amount of heat is generated between p-type gallium nitride and the transparent electrode, shortening the life of a device and impairing the reliability in the operation of the device.